Schedule III - Real Estate and Accumulated Depreciation - Changes in Real Estate Rollforward (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Reconciliation of Carrying Amount of Real Estate Investments [Roll Forward]
Balance at the beginning of period	$ 5,805.4	$ 5,624.2	$ 4,067.1
Additions during the period:
Acquisitions through foreclosure	0.0	0.0	99.3
Other acquisitions	179.5	542.7	1,473.1
Improvements	615.7	366.9	168.1
Consolidations	0.0	39.3	602.1
Foreign currency	426.9	(423.6)	(270.3)
Deductions during the period:
Cost of real estate sold	(448.9)	(344.1)	(515.2)
Balance at close of period	$ 6,578.6	$ 5,805.4	$ 5,624.2